Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from a substantially cylindrical actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of an array of permanent magnets. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the permanent magnets and causes the coil to move relative to the permanent magnets in accordance with the well-known Lorentz relationship. As the coil moves relative to the permanent magnets, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Typically, the heads are supported over the discs by actuator slider assemblies which include air-bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the discs, so that the heads are said to "fly" over the disc surfaces. Generally, the heads write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time-varying write current to the head. In order to subsequently read back the data stored on the data track, the head detects flux transitions in the magnetic fields of the data track and converts these to a signal which is decoded by read channel circuitry of the disc drive.
Of the variety of head constructions presently used in modern disc drives, magneto-resistive heads (hereinafter "MR heads") are of particular interest. MR heads include the use of a thin film element which has the characteristic of having a changed DC resistance in the presence of a magnetic field of a predetermined orientation. Taking advantage of this characteristic, MR heads generally perform a read operation by passing a bias current through this thin film element and detecting changes in this bias current as a result of exposure of the thin film element to the flux transitions from the data track (as the resistance of the thin film element is changed thereby). These changes in the bias current are subsequently decoded by the read channel in order to reconstruct the data that was previously stored on the track. Generally, the MR heads write data to the track using an inductive writing technique, whereby a write current of a selected magnitude and polarity is passed through the head in order to selectively magnetize portions of the data track.
Typically, MR heads have two pairs of electrical connections, one pair for the write element portion of the head and one pair for the read element portion of the head, with each pair of connections comprising a current source and a current return path. Thus, during a read operation, one pair of the connections is used to pass the read bias current through the thin film element of the MR head and time-varying changes in the read bias current are detected by AC sense circuitry; during a write operation, the other pair of connections is used to provide the write current to the head in order to selectively magnetize the data track.
It is well known that the utilization of MR heads has led to further improvements in data storage capabilities of modern disc drives, in that increased areal densities (that is, the number of storage elements per square unit of disc surface, presently quantified in megabits per square inch) have been achieved partially in response to the use of such MR heads. As market forces continue to provide economic incentive for pushing the areal density design envelope, it is increasingly desirable to optimize the performance of MR heads in new disc drives.
Performance problems have been encountered, however, in these efforts to continually increase areal density. Because of a variety of factors encountered in modern disc drives, it is generally desirable to vary both the write current and the read bias current, both from drive to drive and with respect to location of the head over the disc surface. Such factors influencing the read and write performance of the drives include, for example, variations in the flight height of the heads with respect to the disc radius, changes in the linear velocity of the disc with respect to disc radius, the skew of the slider with respect to the data track, the throat height of the head, changes in data transfer rates with respect to disc location and general head/media tolerances inherent in high volume disc drive manufacturing processes.
Efforts have been made in the prior art to optimize currents passed through heads to address these affects; see, for example, U.S. Pat. No. 4,799,112 entitled METHOD AND APPARATUS FOR RECORDING DATA, issued Jan. 17, 1989 to Bremmer et al. (which discloses zone based recording) and U.S. Pat. No. 5,107,378 entitled ADAPTIVE MAGNETIC RECORDING AND READBACK SYSTEM, issued Apr. 21, 1992 to Cronch et al. (which discloses write current optimization). Both these are assigned to the assignee of the present application and are incorporated herein by reference.
However, with continued efforts to increase areal density in disc drive designs, there remains a need for an improved approach to dynamically control the write and read bias currents in a disc drive employing the use of MR heads.